SOIL GAS AS A MONITORING TOOL OF DEEP GEOLOGICAL SEQUESTRATION OF CARBON DIOXIDE: RESULTS FROM THE THREE YEAR MONITORING OF THE ENCANA EOR PROJECT, WEYBURN, SASKATCHEWAN (CANADA)

摘要

As part of the IEA Weyburn CO_2 Monitoring and Storage Project, preliminary baseline soil gas data was collected in the summer and autumn of 2001 above the Phase 1A injection area of the EnCana Enhanced Oil Recovery project at the Weyburn oilfield in south Saskatchewan. The 2001 data was presented at GHGT-6 in Kyoto. Data can now be presented for all three years of the study with conclusions, the predominant one being that the major controls on soil gas levels are seasonal and meteorological with no indications of leakage from depth.rnIn the autumns of 2002 and 2003 further in situ monitoring of CO_2, CO_2 flux, O_2, CH_4, radon (~(222)Rn) and thoron (~(220)Rn) was carried out. Soil gas samples were also collected for laboratory analysis of helium, permanent gases, sulphur species and light hydrocarbons. All sampling was repeated over the same 360 point sampling grid and more detailed profiles for both follow-up years. Marked changes in CO_2 levels (especially flux) for each of the three-year datasets indicate changes in surface conditions, rather than CO_2 from a deeper source. The radon and thoron data was found to be similar for the three years but appeared to vary in response to drift composition, and seasonal effects, rather than migration from a deep source.rnIn 2003 further work was agreed in addition to collecting the main grid and profile data. A control area was sampled for the same suite of gases 10km to the northwest of the oil field. It included similar topography, land use and drift composition to the main sampling grid. There were 35 sample locations on a 7 x 5 point grid with 100m spacing and two additional sites. Early conclusions indicated that the soil gas results in the control area were very similar to those from the main grid on the oilfield suggesting that there was no elevated CO_2 in the soil covering the injection area and this further supported the suggestion that there is a lack of deeply sourced CO_2.rnAlong with the control site, five zones of possible CO_2 leakage were also surveyed and sampled. Two profiles across a river lineament that may be associated with deep faulting, two abandoned oil well sites and one site which overlays a deep salt dissolution feature. (Unfortunately CO_2 flux and gamma measurements were not carried out at these sites.).rnNortheast to southwest trending lineaments, just north of the main grid and generally following an incised river valley displayed anomalous CO_2 only on the valley floor where it would be expected as there was lush vegetation in this zone. Apart from one helium anomaly being coincident with a CO_2 high in an area of low terrain close to a creek there were no other coincident anomalies for other gases. Borehole integrity was investigated by measuring soil gas around two abandoned oil wells. A 16-site grid was surveyed around each well. One well had been completely abandoned and the other was suspended due to failed casing. The well with failed casing had weakly anomalous CO_2 at two sites but this was unmatched for other gases. The fully abandoned well had background CO_2 values. Two perpendicular profiles of 10 sites at 25m spacing were sampled for soil gas over the mapped centre of the dissolution feature. Background values were obtained.rnIn 2003 two vertical profiles were performed. Although there were no radon anomalies both profiles demonstrated an increase in CO_2 to a maximum depth of 1.80m. This increase is matched by a corresponding decrease only in O_2 indicating biological respiration.